High frequency simultaneous metrics antenna (HF-SIMANT)

ABSTRACT

An antenna comprising: a loop made of conductive material; two baluns connected to, and intersecting, opposing sides of the loop, wherein each balun has an output; a 180° hybrid coupler having two input ports, a sum output port, and a delta output port, wherein the two input ports are connected to the outputs of the baluns; a first low noise amplifier (LNA) connected to the sum output port; a second LNA connected to the delta output port; first and second receivers connected to the first and second LNAs respectively; and wherein the antenna is electrically small and is designed to simultaneously receive wideband signals in real time from 3 to 30 MHz.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of prior U.S. applicationSer. No. 15/263,550, filed 13 Sep. 2016, titled “Six Degrees of FreedomGround Exploiting Vector Sensor Antenna (6Ge Antenna)” (Navy Case#102566), which application is hereby incorporated by reference hereinin its entirety for its teachings, and referred to hereafter as “theparent application.”

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

The United States Government has ownership rights in this invention.Licensing and technical inquiries may be directed to the Office ofResearch and Technical Applications, Space and Naval Warfare SystemsCenter, Pacific, Code 72120, San Diego, Calif., 92152; voice (619)553-5118; ssc_pac_t2@navy.mil. Reference Navy Case Number 102502.

BACKGROUND OF THE INVENTION

The antenna disclosed herein relates to the field of High Frequency (HF)communications and direction-finding system applications. Previoussystems for direction-finding and HF communications have attempted tomeet the following requirements with varying degrees of success:simultaneous dipole and loop mode performance, low noise, electricallysmall and wideband operation from 3 to 30 MHz. Some prior art systemsuse many antenna structures operating in different modes, and in manyimplementations, covering different parts of the 3 to 30 MHz spectrum tomeet these requirements. Problems encountered by prior art systemsinclude, among other things, installation difficulty, balun designdifficulty, and poor antenna performance.

SUMMARY

Described herein is an antenna comprising: a loop, two baluns, a 180°hybrid coupler, first and second low noise amplifiers (LNAs), an firstand second receivers. The loop is made of conductive material. The twobaluns are connected to, and intersect, opposing sides of the loop. Eachbalun has an output. The 180° hybrid coupler has two input ports, a sumoutput port, and a delta output port. The two input ports are connectedto the outputs of the baluns. The first LNA is connected to the sumoutput port. The second LNA is connected to the delta output port. Thefirst and second receivers are connected to the first and second LNAsrespectively. The antenna described herein and claimed hereafter iselectrically small and is designed to simultaneously receive widebandsignals in real time from 3 to 30 MHz.

An embodiment of the antenna may be described as comprising first,second, and third loop antennas. In this embodiment of the antenna, eachof the first, second, and third loop antennas is similar to the antennadescribed above. The loops of the first, second, and third loop antennasare disposed in mutually orthogonal planes and are positioned such thatthey do not touch each other. This embodiment of the antenna mayfunction as a HF direction-finding antenna that is capable ofsimultaneously operating in dipole and loop mode and is capable ofsimultaneously receiving wideband signals in real time from 3 to 30 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the several views, like elements are referenced using likereferences. The elements in the figures are not drawn to scale and somedimensions are exaggerated for clarity.

FIG. 1 is a perspective view of a general pictorial of a high frequencysimultaneous metrics antenna.

FIG. 2 is a block diagram of an embodiment of a high frequencysimultaneous metrics antenna.

FIG. 3 is a block diagram of an embodiment of a high frequencysimultaneous metrics antenna.

FIG. 4A is a perspective view of an embodiment of a high frequencysimultaneous metrics antenna.

FIG. 4B is an illustration of a coordinate plane.

FIGS. 5A-5B are plots of polarization responses of a high frequencysimultaneous metrics antenna in loop mode.

FIGS. 5C-5D are plots of polarization responses of a high frequencysimultaneous metrics antenna in dipole mode.

FIG. 6 is a block diagram of an embodiment of a high frequencysimultaneous metrics antenna.

FIGS. 7A-7B are plots of antenna patterns of a high frequencysimultaneous metrics antenna in loop mode.

FIG. 8 is a plot of calculated system noise figures.

FIG. 9 is a perspective view of a general pictorial of an octagonal highfrequency simultaneous metrics antenna.

FIG. 10A is a circuit diagram.

FIG. 10B is a photo of an example balun.

FIG. 11 is a cross-sectional, side-view of an example balun.

FIG. 12 is a perspective view of a general pictorial of avector-sensing, three-orthogonal-loop, high frequency simultaneousmetrics antenna.

FIG. 13 is a photo of an antenna loop cross-over section.

FIG. 14 is a photo of an example coaxial interface.

FIG. 15 is a photo of a vector-sensing prototype of a high frequencysimultaneous metrics antenna.

FIG. 16 is a circuit diagram.

DETAILED DESCRIPTION OF EMBODIMENTS

The disclosed antenna below may be described generally, as well as interms of specific examples and/or specific embodiments. For instanceswhere references are made to detailed examples and/or embodiments, itshould be appreciated that any of the underlying principles describedare not to be limited to a single embodiment, but may be expanded foruse with any of the other antenna or method related thereto describedherein as will be understood by one of ordinary skill in the art unlessotherwise stated specifically.

FIG. 1 is a side view illustration of an embodiment of a High FrequencySimultaneous Metrics Antenna 10 (referred to hereafter as the HF-SIMANT10). The HF-SIMANT 10 comprises, consists of, or consists essentially ofa conductive loop 12, two baluns 14 ₁ and 14 ₂, a 180° hybrid coupler16, first and second low noise amplifiers (LNAs) 18 ₁ and 18 ₂, an firstand second receivers 20 ₁ and 20 ₂. The two baluns 14 ₁ and 14 ₂ areconnected to, and intersect, opposing sides of the loop 12. Each balun14 has an output 22. The 180° hybrid coupler has two input ports 24 ₁and 24 ₂, a sum output port 26, and a delta output port 28. The twoinput ports 24 ₁ and 24 ₂ are connected to the outputs of the baluns 22₁ and 22 ₂. The first LNA 18 ₁ is connected to the sum output port 26.The second LNA 18 ₂ is connected to the delta output port 28. The firstand second receivers 20 ₁ and 20 ₂ are connected to the first and secondLNAs 18 ₁ and 18 ₂ respectively. The HF-SIMANT 10 is electrically smalland is designed to simultaneously receive wideband signals in real timefrom 3 to 30 MHz.

The radius r of the loop 12 is a design variable that may be adjusteddepending on the desired performance range. For example, the radius r ofthe loop 12 may be 0.5 meters for suitable wideband (i.e., 3 to 30 MHz)performance. The shape of the loop 12 is also a design variable. Theloop 12 may be circular, octagonal, rectangular, etc. The loop 12 may bemade of any conductive material and may be solid or hollow. For example,in one embodiment, the loop 12 may be fabricated as an octagon usingcopper tubing having a diameter of approximately 1.5 cm (0.6 inches).

FIG. 2 is a detailed block diagram of an example embodiment of theHF-SIMANT 10. In this embodiment shown in the block diagram, each balun14 is connected to a transmission line 30 of approximately 0.5 meterslength. The transmission loss for the transmission line may be expectedto be approximately 0.1 dB. Both transmission lines 30 ₁ and 30 ₂ areconnected into the 180-degree hybrid coupler 16. The output of the SUM(S_(E)) port 26 is the sum of the two input signals. The output of theDELTA (I_(N)) port 28 is the difference of the two input signals. Inthis embodiment of the HF-SIMANT 10, both the SUM and DELTA outputs arefollowed by coaxial cables 32 ₁ and 32 ₂, both of which measure 2 metersin length, and are followed by the low noise LNAs 18 ₁ and 18 ₂.Suitable examples of the LNAs 18 ₁ and 18 ₂ include, but are not limitedto, the Shireen LNA-643 with a Noise Figure of 2.5 dB, and an IP3 of 35dBm and a gain of 41 dB. Then, in this embodiment, both LNAs 18 ₁ and 18₂ are then followed by 30 meter cables 34 ₁ and 34 ₂ with approximately1.4 dB of loss. The SUM signal and the DELTA signal are then inputs toreceivers 20 ₁ and 20 ₂ where 8 dB NF may be assumed. Each of thereceivers 20 ₁ and 20 ₂ may be any radio receiver that operates at HF. Asuitable, non-limiting, example of the receiver 20 is a RFSPACE SDR-IPsoftware-defined radio. At a bare minimum, each of the receivers 20 ₁and 20 ₂ would need to have a mixer, a low pass filter to down-convert asignal to baseband, and an analog to digital converter. The specificvalues described above and shown in FIG. 2 were selected to meet severalperformance metrics of an HF receive antenna. These performance metricsinclude dual-mode (i.e., both dipole and loop), wideband (i.e., 3 to 30MHz), low noise, and electrically small (i.e., electrically smaller thanthe highest operating frequency of 30 MHz). Other specific values may beused for other desired performance and/or antenna size. The baluns 14may be any desired size and have any desired value depending on thedesired performance. For example, in the embodiment of the HF-SIMANT 10shown in FIG. 1, the baluns are 1:1 Transmission Line Transformers(TLTs) that use 50Ω twisted-pair transmission lines. Still referring tothe same embodiment, each balun will add phase delay associated with theelectrical length of the twisted-pair transmission line; the phase delaythrough the balun 14 was measured and fit to a straight line(ϕ_(Balun)=−0.55 f_(MHz)). Another suitable embodiment of the balun 14includes, but is not limited to, a 9:1 balun.

FIG. 3 is a detailed block diagram of another example embodiment of theHF-SIMANT 10. This embodiment of the HF-SIMANT 10 further comprisesfirst and second matching networks 36 ₁ and 36 ₂, which are respectivelyconnected in parallel to the sum output port 26 and the delta outputport 28. The matching networks 36 ₁ and 36 ₂ may be any broadbandmatching network capable of balancing source and load impedances. In theembodiment of the HF-SIMANT 10 shown in FIG. 3, the matching networks 36₁ and 36 ₂ are capacitors, each capacitor having two terminals, one ofwhich is grounded and the other is connected to its respective outputport of the 180° hybrid coupler 16.

FIG. 4A is a perspective illustration of a vector sensing embodiment ofthe HF-SIMANT 10 comprising first, second, and third loop antennas 40 ₁,40 ₂, and 40 ₃ that are orthogonal to each other and are disposed abovea ground plane 38. In this embodiment, each of the first, second, andthird loop antennas 40 ₁, 40 ₂, and 40 ₃ is similar to the embodiment ofthe HF-SIMANT 10 shown in FIG. 1. The loops of the first, second, andthird loop antennas 40 ₁, 40 ₂, and 40 ₃ are disposed in mutuallyorthogonal planes and are positioned such that they do not touch eachother. This embodiment of the HF-SIMANT 10 may function as a HFdirection-finding antenna that is capable of simultaneously operating indipole and loop mode and is capable of simultaneously receiving widebandsignals in real time from 3 to 30 MHz. The three loop antennas 40 ₁, 40₂, and 40 ₃ are evenly spaced in azimuth around a vertical axis 42 andprovide excellent dual-mode (i.e., dipole and loop) performance. FIG. 4Bshows a received signal 44 and a three-dimensional coordinate plane. Theangle phi Φ is shown in FIG. 4B.

FIGS. 5A-5D are plots of polarization responses of the vector sensingembodiment of the HF-SIMANT 10 shown in FIG. 4A in loop mode at 3 MHz.FIG. 5A is a plot of the vertical polarization response and FIG. 5B is aplot of the horizontal polarization response of the aforementionedembodiment. FIGS. 5C-5D are plots of polarization responses of thevector-sensing embodiment of the HF-SIMANT 10 shown in FIG. 4A in dipolemode at 3 MHz. FIG. 5C is a plot of the vertical polarization responseand FIG. 5D is a plot of the horizontal polarization response of theaforementioned embodiment. Theta θ is the zenith angle of the receivesignal 40. Phi Φ is the azimuthal angle measured from the x-axis of thereceive signal 44. The radiation pattern in FIGS. 5A-5B is the receivedirectivity of the vector sensing embodiment of the HF-SIMANT 10 shownin FIG. 4A in loop mode. The radiation pattern in FIGS. 5C-5D is thereceive directivity of the vector sensing embodiment of the HF-SIMANT 10shown in FIG. 4A in dipole mode. The receive frequency in FIGS. 5A-5Dwas 3 MHz. The HF-SIMANT 10 is capable of simultaneously supporting thereception of both the dipole and loop modes.

The HF-SIMANT 10 may be designed to keep its system noise below that ofexternal high frequency (HF) noise. Because the HF-SIMANT 10 isprimarily designed to receive signals it can tolerate inefficient,mismatched antennas if the internal system noise is much lower thanexternal environmental noise, because the small antenna will reject thedesired signal and in-band external noise equally. Sources of HFexternal noise are galactic noise, atmospheric noise, and man-madenoise. Galactic noise from space is usually much lower power thanatmospheric noise, which is primarily from lightning strikes. Majorsources of man-made noise are engines and power distribution equipment,and noise power varies by location. The International TelecommunicationUnion categorizes locations by their expected noise level into QuietRural, Rural, Urban, and Industrial. The embodiment of the HF-SIMANT 10shown in FIG. 4A is designed to have a system noise level less than theexpected noise level of a Quiet Rural environment, over the HF band. Theantenna has a system noise figure that is less than quiet rural noiselevels set by the International Radio Consultative Committee (CCIR).

The system noise level may be computed assuming a somewhat simplifiedradio frequency (RF) receive chain such as represented by the blockdiagram shown in FIG. 6. The computation neglects bias tees andtransient protection devices because these components should notintroduce significant loss or mismatch at HF and thus contribute littleto the system noise level. The components that will set the system noiselevel are the loops 12, the baluns 14, the hybrid coupler 16, the LNAs18, the receivers 20, and the cables 30, 32, and 34. The noise figure NFis given by:

$\begin{matrix}{{NF}_{i} = {10\;{\log_{10}\left( {1 + \frac{T_{i}}{T_{0}}} \right)}{dB}}} & (1)\end{matrix}$where i=Σ or Δ corresponding to the noise figure of the dipole or loopmode, respectively. The mismatch factor through the 180° hybrid coupler16 is different for the dipole and loop modes, as will be shown next, sothe system noise figure is slightly different. T₀ is the standard noisetemperature, 290° K. The system noise temperature, T_(i), is given by

$\begin{matrix}{T_{i} = {T_{0}\left( {\frac{1 - \eta_{r}}{\eta_{r}} + \frac{1 - G_{{cable}\mspace{11mu} 1}}{\eta_{r}\tau_{feedpoint}G_{{cable}\mspace{11mu} 1}} + \frac{1 - G_{hybrid}}{\eta_{r}\tau_{feedpoint}G_{{cable}\mspace{11mu} 1}G_{hybrid}} + \frac{1 - G_{{cable}\mspace{11mu} 2}}{\eta_{r}\tau_{feedpoint}G_{{cable}\mspace{11mu} 1}G_{hybrid}\tau_{i}G_{{cable}\mspace{11mu} 2}} + \frac{f_{LNA} - 1}{\eta_{r}\tau_{feedpoint}G_{{cable}\mspace{11mu} 1}G_{hybrid}\tau_{i}G_{{cable}\mspace{11mu} 2}} + \frac{1 - G_{{cable}\mspace{11mu} 3}}{\eta_{r}\tau_{feedpoint}G_{{cable}\mspace{11mu} 1}G_{hybrid}\tau_{i}G_{{cable}\mspace{11mu} 2}G_{LNA}G_{{cable}\mspace{11mu} 3}} + \frac{f_{RX} - 1}{\eta_{r}\tau_{feedpoint}G_{{cable}\mspace{11mu} 1}G_{hybrid}\tau_{i}G_{{cable}\mspace{11mu} 2}G_{LNA}G_{{cable}\mspace{11mu} 3}}} \right)}} & (2)\end{matrix}$where the following variables are defined going from the conductive loop12 to the receiver 20:η_(r) is the antenna radiation efficiency;τ_(feedpoint) is the mismatch loss after the baluns 14;G_(cable1) is the gain (less than unity) of the cable connecting thebalun 14 to the 180° hybrid coupler 16;G_(hybrid) is the gain (less than unity) through the 180° hybrid coupler16;τ_(i) is the hybrid port i mismatch loss;G_(cable2) is the gain (less than unity) of the cable connecting the180° hybrid coupler 16 to the LNA 18;f_(LNA) is the noise factor of the LNA 18;G_(LNA) is the gain of the LNA 18;G_(cable3) is gain of the cable connecting the LNA 18 to the receiver20; andF_(RX) is the noise factor of the receiver 20.

The antenna radiation efficiency may be supplied by computersimulations, and the mismatch may be from the measured S-parameters. Anexample of a suitable program for providing computer simulations isMININEC Pro Antenna Analysis Software. The attenuation from cables 30,32, and 34, the gain of the LNA 18, and the noise factor of the LNA 18and receiver 20 can be obtained from manufacturer specifications. Theincoming voltage waves (v⁺) into the 180° hybrid coupler 16 and outgoingvoltage waves (v⁻) are related to the S-parameters by:

$\begin{matrix}{\begin{bmatrix}v_{1}^{-} \\v_{2}^{-} \\v_{3}^{-} \\v_{4}^{-}\end{bmatrix} = {S_{hybrid}\begin{bmatrix}v_{1}^{+} \\v_{2}^{+} \\v_{3}^{+} \\v_{4}^{+}\end{bmatrix}}} & (3)\end{matrix}$The 180° hybrid coupler 16 may be assumed to be ideal since at HF itsloss is negligible. The S-parameters of the 180° hybrid coupler 16 aregiven by:

$\begin{matrix}{S_{hybrid} = {\frac{- j}{\sqrt{2}}\begin{bmatrix}0 & 1 & 1 & 0 \\1 & 0 & 0 & {- 1} \\1 & 0 & 0 & 1 \\0 & {- 1} & 1 & 0\end{bmatrix}}} & (4)\end{matrix}$Equation 4 may be used to determine the hybrid coupler sum anddifference ports mismatch losses in the following manner. Let theS-parameter matrix S_(composite) consist of the two-port loop antennashown in FIG. 6 including everything from a conductive loop 12 up to thetwo coaxial cables 30 that connect to the 180° hybrid coupler 16. Thesame voltage waves defined for the 180° hybrid coupler 16 in equation 3are now related to this S_(composite) two-port by:

$\begin{matrix}{\begin{bmatrix}v_{2}^{+} \\v_{3}^{+}\end{bmatrix} = {{S_{composite}\begin{bmatrix}v_{2}^{-} \\v_{3}^{-}\end{bmatrix}} = {\begin{bmatrix}s_{11} & s_{12} \\s_{21} & s_{22}\end{bmatrix}\begin{bmatrix}v_{2}^{-} \\v_{3}^{-}\end{bmatrix}}}} & (5)\end{matrix}$The reflection coefficients looking into the sum and difference ports(26 and 28 respectively) of the 180° hybrid coupler 16 are given by thefollowing two equations:

$\begin{matrix}{\Gamma_{{in},\sum} = {\frac{v_{1}^{-}}{v_{1}^{+}}❘_{v_{4}^{+} = 0}}} & (6) \\{\Gamma_{{in},\Delta} = {\frac{v_{4}^{-}}{v_{4}^{+}}❘_{v_{1}^{+} = 0}}} & (7)\end{matrix}$These may be written in terms of the S-parameters of the compositetwo-port connected to the 180° hybrid coupler 16:

$\begin{matrix}{\Gamma_{{in},\sum} = {{- \frac{1}{2}}\left( {s_{11} + s_{12} + s_{21} + s_{22}} \right)}} & (8) \\{\Gamma_{{in},\Delta} = {\frac{1}{2}\left( {{- s_{11}} + s_{12} + s_{21} - s_{22}} \right)}} & (9)\end{matrix}$The mismatch factors at the sum and difference ports (22 and 24respectively) of the 180° hybrid coupler 16 may be computed by:τ_(Σ)=1−|Γ_(in,Σ)|²  (10)τ_(Δ)=1−|Γ_(in,Δ)|²  (11)All of these inputs may be used to calculate the system noisetemperature in Equation 2 which may be used in Equation 1 to calculatethe system noise figure.

FIGS. 7A and 7B are plots showing the measured antenna pattern for anoctagonal embodiment of the HF-SIMANT 10 having a circumference ofapproximately 3.4 meters. This embodiment of the HF-SIMANT 10 is a smallantenna, because the circumference is 0.34 wavelengths at the highestoperating frequency of 30 MHz. The pattern shown in FIG. 7A. is for avertical orientation of the antenna, with the pattern in the plane ofthe two feed points (baluns), and vertical polarization at 30 MHz. InFIG. 7A, the upper trace corresponds to the delta port and the lowertrace corresponds to the sum port. The pattern in FIG. 7B is for ahorizontal orientation of the antenna, with the pattern in the plane ofthe loop 12, and horizontal polarization at 30 MHz. In FIG. 7B, theupper trace corresponds to the sum port and the lower trace correspondsto the delta port.

FIG. 7A shows the classic antenna pattern for a small loop antenna. FIG.7B shows the classic antenna pattern for a small dipole antenna.HF-SIMANT 10 is a small antenna at 30 MHz. Therefore, as the frequencyis decreased to 3 MHz, the antenna pattern structure will be maintained.The directivity of the antenna pattern does not change with frequency.The measurements also confirm that both a dipole and loop mode areavailable using an appropriate two-port feed system such as is shown bythe baluns 14 in FIGS. 1-3. The dipole and loop modes are maintainedfrom 3 to 30 MHz.

FIG. 8 is a plot of calculated system noise figures of sum (dipole mode)and difference (loop mode) of the octagonal embodiment of the HF-SIMANT10 having a circumference of approximately 3.4 meters compared to theQuiet Rural HF noise model. Well-designed HF receive systems areexternal noise limited. In other words, the Noise Figure is maintainedbelow the Noise Figure for a given noise environment. As shown by FIG.8, this embodiment of the HF-SIMANT 10, in both the dipole and loopmode, has a Noise Figure below the quiet rural Noise Figure (below 20MHz) and only slightly above that for frequencies higher than 20 MHz.The graph in FIG. 8 shows that the expected noise in a Quiet Ruralenvironment is higher than the system noise figure for both the loop anddipole synthesized antenna patterns.

FIG. 9 is a general pictorial of an embodiment of the loop antenna 40.The loop antenna 40 with its two ports (baluns 14) has some interestingand unique properties. As discussed above, since the two ports arediametrically opposed, the loop can be made to operate in two modes. Thedifference of the two port voltages results in constant current flowaround the loop 12. This generates a magnetic dipole moment (I_(m))orthogonal to the plane of the loop. Similarly, the sum of the two portvoltages results in current flow in opposite directions around the loop.In this case, the currents cancel each other at the top and bottom ofthe loop generating an electric dipole moment (I_(e)) in the plane ofthe loop and orthogonal to the two ports. The adding and subtracting ofthese port voltages for each loop antenna is effectively accomplishedusing the 180° hybrid coupler 16. Placing three of these loopsorthogonal to each other results in three orthogonal electric dipolemoments and three orthogonal magnetic dipole moments. The same matchingmay be used for the loop and dipole modes. Note that in the HF-SIMANT 10there are no long dipoles protruding through the loops.

FIG. 10A is a circuit model of a 9:1 embodiment of the balun 14, anexample of which is shown in FIG. 10B. In this embodiment, the balun 14consists of three twisted-pair transmission lines 46 wrapped on threeferrite toroids 48 with the leads connected as shown in FIG. 10A. Thisembodiment of the balun 14 is intended to be used with a loop 12 made ofcopper tubing, but it is to be understood that this is only offered asan example embodiment and that the HF-SIMANT is not limited to theembodiment of the balun 14 described hereafter. The toroid diameter maybe selected so the balun 14 may fit inside the loop 12 (e.g., when theloop 12 is made of tubing). The balun 14 may be attached to the end of afeed coax line 50 (shown in FIG. 11) and may be potted with epoxy toprevent stress on either the coax or the balanced twin-leads. This alsoserves to seal the balun from moisture.

FIG. 11 is an internal side-view of an embodiment of the balun 14. Inthis embodiment, a metal collar 52 may be included and potted so theassembly can be secured to the tubing. This prevents the balun assemblyfrom moving out of position. Third, a high-temperature sleeve 54 may beused to cover the outside of the coax and may be potted with the metalcollar 52 of the balun assembly. A suitable material for the sleeve 54is Ultra Temp 391TM-2-5 by Cotronics Corporation, which material iswoven from continuous filament Alumina ceramic fibers that can withstandcontinuous temperatures of 2600° F. The sleeve 54 may be included sothat some of the copper tubing could be soldered with the coax in placewithout causing damage to the coaxial cable.

FIG. 12 is an illustration of an embodiment of the HF-SIMANT 10. Note inthis embodiment, the feed cross-over sections, one of which is indicatedby circle 56. It is seen in FIG. 12 that one of the other loop antennas40 crosses at each one of the feed points. Therefore, it is desirable todesign this cross-over section so that the conducting section of theother loop antenna 40 does not negatively impact the performance of thefeed point. The conducting part of this design may be a wide copperstrap soldered to a gap in the tubing. This provides a good groundconnection to the conductive loop section and provides room for the feedpoint connection. The strap may be kept wide to minimize the inductanceof the connection.

FIG. 13 shows a photo of an inside view of the above-discussedcross-over design. The cross-over section may be fastened together withtwo insulating blocks 58, one of which is shown in FIG. 13. A suitablematerial for the insulating blocks 58 is Delrin, which is what isdepicted in FIG. 13. The insulating blocks 58 are used to support thecopper tubing of the two crossing loop antennas 40 and may be sealedtogether with any desired fastener or adhesive, such as gasket sealer.

FIG. 14 is a photo of an optional coaxial cable interface 60. TheHF-SIMANT 10 may include a separate coaxial cable interface. A separatecoaxial interface is desirable since the baluns and cables within eachloop antenna 40 are difficult to replace. For example, with respect tothe three-orthogonal-copper-octagonal-loop embodiment of the HF-SIMANT10 discussed above, the copper tubing would have to be soldered afterthe baluns 14 are installed. Therefore, if a cable is damaged, much ofthe antenna would need to be taken apart to repair the damage. Toprevent this, the feed point cables 62 for each loop may be terminatedin a plastic box 64 and attached to bulkhead connectors 66.

FIG. 15 is a photo of a prototype three-orthogonal-copper-octagonal-loopembodiment of the HF-SIMANT 10 similar to that described above. As muchof the tubing and elbows were soldered together as possible. This helpedto keep each loop as close to octagonal and flat as possible. Theantenna was assembled in sections using the feed point connections tokeep the loops flat and orthogonal. Finally, baluns were installed andthe remaining tubing soldered. The coax cables ran on the inside of thecopper tubing and exited through copper T-sections soldered into thebottom of each loop. This prototype of the HF-SIMANT 10 was mounted to anon-conductive PVC plate (not visible in FIG. 15) such that theT-sections were mounted through the PVC plate. These holes were thensealed and the loops were attached to the PVC plate using insulatingbrackets (also not visible in FIG. 15), which in this embodiment werealso made of Delrin. The coaxial cable interfaces 60, one for each loopantenna 40, were mounted to the underside of the PVC plate.

The RF components of the HF-SIMANT 10, such as the 180° hybrid couplers16 and the LNAs 18 may be part of an RF unit 72. FIG. 16 is a blockdiagram of an example embodiment of the RF unit 72 that, in addition tothe hybrids and the LNAs of the basic system block diagram, this unitincludes transient protection devices (TPDs) 74, coaxial switches 76 anda 6:1 power splitter 78 for calibration, and bias-T networks 80 toprovide power to the LNAs 18 and coaxial switches 76. In a prototypeembodiment of the RF unit 72 the following parts were used: TPDs fromFischer Custom Communications (FCC-550-5-SMA), hybrids from UniversalMicrowave Components Corporation (HC-W200-MS), coaxial switches fromMini-Circuits (MSP2TA-18-12+), power splitter from Mini-Circuits(ZFSC-6-1-S(+)), and LNAs from Shireen (LNA-643). One pair of bias-Ts isused to power the six LNAs. This required the bias-T network to provide0.81 amps at 12 volts. Another pair of bias-Ts is used to power thecoaxial switches. This required the bias-T network to provide up to 2.6amps at 12 volts. A common 3 amp bias-T network was designed. To providethis function, a 3 amp HF bias-T network design was identified. Thisdesign was modeled using LTspice and it was determined that the risetime of the step response needed to be slowed to prevent damage to theLNAs. This was accomplished by including an RC network on each of the DCports.

From the above description of the HF-SIMANT 10, it is manifest thatvarious techniques may be used for implementing the concepts of theantenna without departing from the scope of the claims. The describedembodiments are to be considered in all respects as illustrative and notrestrictive. The method/apparatus disclosed herein may be practiced inthe absence of any element that is not specifically claimed and/ordisclosed herein. It should also be understood that the HF-SIMANT 10 isnot limited to the particular embodiments described herein, but iscapable of many embodiments without departing from the scope of theclaims.

We claim:
 1. An antenna comprising: a loop made of conductive material;two baluns connected to, and intersecting, opposing sides of the loop,wherein each balun has an output; a 180° hybrid coupler having two inputports, a sum output port, and a delta output port, wherein the two inputports are connected to the outputs of the baluns; a first low noiseamplifier (LNA) connected to the sum output port; a second LNA connectedto the delta output port; first and second matching networksrespectively connected in parallel to the sum output port and the deltaoutput port, wherein the first and second matching networks arecapacitors, each capacitor having two terminals, one of which isgrounded and the other is connected to its respective output port of the180° hybrid coupler; and first and second receivers connected to thefirst and second LNAs, respectively, wherein the antenna sets a systemnoise level below external high frequency noise by utilizing acomputation for a system noise figure that ignores bias tees andtransient protection devices.
 2. The antenna of claim 1, wherein thefirst matching network has a value of 200 pF and the second matchingnetwork has a value of 150 pF.
 3. The antenna of claim 2, wherein eachof the balun outputs is connected to the 180° hybrid coupler via arespective 0.5 m cable, wherein the sum and delta output ports arerespectively connected to the first and second LNA via first and second2 m cables, and the first and second LNAs are respectively connected tothe first and second receivers via first and second 30 m cables.
 4. Theantenna of claim 1, wherein the loop is octagonal wherein the baluns arecentered on opposing sides of the octagon.
 5. The antenna of claim 4,wherein the loop is made of copper tubing having a loop radius of nogreater than 0.54 m and a tubing diameter of no greater than 1.52 cm(0.6 in).
 6. The antenna of claim 5, wherein the system noise figure isless than quiet rural noise levels set by the International RadioConsultative Committee (CCIR).
 7. The antenna of claim 1, furthercomprising second and third antennas each of which having the samecomponents as the antenna of claim 1, wherein the three antennas aredisposed in mutually orthogonal planes and are positioned such that theydo not touch each other, such that the three antennas together form avector-sensing, high frequency (HF) direction-finding antenna.
 8. A highfrequency (HF) direction-finding antenna comprising: first, second, andthird loop antennas, each of which comprising: a loop of conductivematerial, wherein the loops of the first, second, and third loopantennas are disposed in mutually orthogonal planes and are positionedsuch that they do not touch each other; two baluns connected to, andintersecting, opposing sides of the loop, wherein each balun has anoutput; a 180° hybrid coupler having two input ports, a sum output port,and a delta output port, wherein the two input ports are connected tothe outputs of the baluns; a first low noise amplifier (LNA) connectedto the sum output port; a second LNA connected to the delta output port;and first and second receivers connected to the first and second LNAs,respectively, wherein the HF direction-finding antenna is capable ofsimultaneously operating in dipole and loop mode, and wherein theantenna sets a system noise level below external high frequency noise byutilizing a computation for a system noise figure that ignores bias teesand transient protection devices, wherein the computation for the systemnoise figure (NF) is:${NF}_{i} = {10{\log_{10}\left( {1 + \frac{T_{i}}{T_{0}}} \right)}{dB}}$wherein i=Σ or Δ corresponding to the noise figure of a dipole or loopmode, respectively, T_(i) is a system noise temperature, and T₀ is astandard noise temperature.
 9. The antenna of claim 8, wherein each loopantenna further comprises first and second matching networksrespectively connected in parallel to the sum output port and the deltaoutput port.
 10. The antenna of claim 9, wherein the first and secondmatching networks are capacitors, each capacitor having two terminals,one of which is grounded and the other is connected to its respectiveoutput port of the 180° hybrid coupler.
 11. The antenna of claim 10,wherein the first matching network has a value of 200 pF and the secondmatching network has a value of 150 pF.
 12. The antenna of claim 11,wherein for each loop antenna each of the balun outputs is connected tothe 180° hybrid coupler via a respective 0.5 m cable, wherein the sumand delta output ports are respectively connected to the first andsecond LNA via first and second 2 m cables, and the first and secondLNAs are respectively connected to the first and second receivers viafirst and second 30 m cables.
 13. The antenna of claim 8, wherein thethree loops are each octagonal wherein the baluns of each loop antennaare centered on opposing sides of the corresponding octagonal loop. 14.The antenna of claim 13, wherein each loop is made of copper tubinghaving a loop radius of no greater than 0.54 m and a tubing diameter ofno greater than 1.52 cm (0.6 in).
 15. The antenna of claim 8, whereinthe system noise figure is less than quiet rural noise levels set by theInternational Radio Consultative Committee (CCIR).
 16. The antenna ofclaim 8, wherein the loops are positioned at least a loop diameter abovea ground plane.
 17. An antenna comprising: a loop made of conductivematerial; two baluns connected to, and intersecting, opposing sides ofthe loop, wherein each balun has an output; a 180° hybrid coupler havingtwo input ports, a sum output port, and a delta output port, wherein thetwo input ports are connected to the outputs of the baluns; a first lownoise amplifier (LNA) connected to the sum output port; a second LNAconnected to the delta output port; and first and second receiversconnected to the first and second LNAs respectively, wherein the antennasets a system noise level below external high frequency noise byutilizing a computation for a system noise figure that ignores bias teesand transient protection devices, wherein the computation for the systemnoise figure (NF) is:${NF}_{i} = {10{\log_{10}\left( {1 + \frac{T_{i}}{T_{0}}} \right)}{dB}}$wherein i=Σ or Δ corresponding to the noise figure of a dipole or loopmode, respectively, T_(i) is a system noise temperature, and T₀ is astandard noise temperature.